I actually think PKW provided a better finale than they would have had if they'd run for the full five seasons. It seemed like they were wrapping things up at the end of season four, and having $20 million to blow on a whopping movie-like series finale worked quite well.
There are a lot of hinted-at and speculated plotlines they had to drop, though (one of the crazier ones is Granny being reincarnated Zhaan... or something).
The thing is, while season four had a couple of missteps; the final arc was still classic Farscape extravagance; expanding and complicating their space opera melodramatics with the internal workings of the Scarran empire; which, who knows, might have had more payoffs later.
I'd agree the whole wormhole macguffin had been beaten to death at this point; but I have confidence that maybe the Farscape writers had some pretty neat stuff lined up for the final two years.
I don't know if that included the Nebari, though I'd agree that'd be nice to see. The thing about the Nebari is, despite their implied importance with Chiana being a regular; they really only featured in two episodes of the show's entire run. Which is, for eample, less often then Durka appeared. Trying to stop their nefarious schemes probably could have been a pretty neat season ender, though.
But admittedly I'm not that sad as Farscape does seem pretty whole in its current form... though I still wanted to see Rygel reclaim his throne, damn it!